1. Field of the Invention
This invention is concerned with chemical vapor deposition processes (CVD) and methods for controlling them. In particular, this invention provides methods for real-time control of the production of a surface coating by deposition from a gaseous phase, through feedback control of the inputs to the production environment.
2. Review of Related Art
Surface modification technologies, by virtue of their ability to tailor the surface properties, have become an important part of materials development. Some of the critical properties that can be engineered by design of the coating morphology and composition include thermal shock resistance, corrosion, wear, and oxidation resistance. Chemical vapor deposition (CVD) has several characteristics that often make it a preferred, and sometimes the only coating method for depositing desired films. CVD is noted for its ability to produce dense coatings from a wide variety of materials, and unlike line-of-sight deposition methods, CVD can produce highly conformal films of uniform thickness on substrates with complex shapes.
The advantages of CVD may be largely offset by the difficulty in developing a set of deposition parameters to reliably deposit films or fabricate components in a manufacturing environment. This is particularly true since, as application requirements become increasingly demanding, several properties--often competing--are required of the coating system. The ability to utilize a monolithic coating may be limited, and novel engineered structures involving multi-layers and multiphases, while potentially providing novel composite materials, are more difficult to manufacture.
To develop a new coating, a research scale reactor is typically used to develop the coating, based on previous processing knowledge and empirical experimentation. Even for relatively simple CVD coatings such as a single component film, a relatively long development time is required. An experimental design for optimization of even a few film variables, such as grain size and film thickness uniformity, leads to a large test matrix and as a result an extensive research period. Scaling-up of the process to a production scale reactor requires a second level of development. Additional time is required to meet the design changes that occur during development of any new product.
Currently, CVD processes are essentially run in a feedforward manner where feedback is used only to maintain specific values of inputs such as flow rates and pressure, but no feedback of in-situ measurements of the growth process is used. Thus, the selection of processing parameters (i.e. input settings such as gas flow rates and temperatures) to achieve materials objectives (e.g. composition and morphology), and processing objectives (e.g. coating layer thickness, growth rate, yield, and reproducibility) have been developed based on previous processing knowledge and empirical experimentation. The difficulty in using such an approach in developing new coatings is the inherent complexity of the process, in part because the process occurs by reaction paths where important constants, such, as the kinetic coefficients, are not known.
Despite its importance and versatility in the deposition of a variety of thin films, significant limitations in developing a generalized understanding of CVD remain. Conventional operation of the process focuses on trial-and-error experiments, guided by insight obtained from extensive scientific and modeling studies that have been performed. The extension of these empirical results to different reactor configurations and to new coatings is generally difficult. Many studies have been aimed at achieving a fundamental understanding of the process and at relating operational conditions--such as pressure, temperature, gas flows and reactor geometry--to performance measures like uniformity, deposition rate, composition and micro structure. Previous work can be classified into three major areas: experimental investigation of the reaction kinetics and coating structures, development of sensors (primarily for diagnostics), and modeling.
Numerous models have been used to obtain insight into the complex nature of CVD. Due to the large computational effort involved in a detailed solution of all relevant equations, many of the proposed models emphasize a particular aspect of CVD. Numerical methods are applied to solve the coupled equations, and they use large numbers of nodes to simulate the process. This is required to capture the coupled nature of the thermal-fluid distribution, chemical reactions, and surface physics.
Early modeling work focused on one-dimensional models that captured the thermal fluid nature of the process. While some of this work focused in analytical solutions and insight (such as that by Giling (de Croon, et al., (1990) J. Electrochem. Soc., 137(9), and Spear (1982) Pure and Applied Chemistry 54(7):1297-1312) it became more complex (i.e. capturing the two- and three-dimensional nature of the process) as computer-based numerical approaches became standard (for example Wahl (1983) Proceedings of the Fourth European Conference on CVD; Wahl (1983) Proceedings of the Ninth International Conference on CVD; Wahl (1983) Proceedings of the Fifth European Conference on CVD); Jensen, et al., (1983) J. Electrochem. Soc., 139(1); Fotiadis, et al., (1990) J. of Cryst. Growth, 100:577-599; and Jensen (1989) Advan. Chem. Ser., 221).
Giling (1990) has articulated a number of the important fluid-thermal phenomena that must be considered in both hot and cold wall reactors. Important issues include laminar/turbulent flow regimes, free convection effects in cold wall reactors, entrance effects, multicomponent diffusivity, boundary layer thickness considerations, and implications for reactor design. This analysis was confirmed by holographic measurements of the temperature and flow distributions.
Spear(1982) focused on developing models that capture mass transport and reactions near the substrate surface. He took into account diffusion of multiple species through the boundary layer, adsorption and reactions. He also pioneered the use of thermodynamic models of steady state CVD reactions, which provide useful insight into the equilibrium phenomena of the CVD process. Thermodynamic studies are important because they provide insight into possible gas phase and surface reactions, but do not take into consideration kinetic limitations that can significantly affect the process.
Wahl (1983) was one of the authors that began to develop more complete representations of the entire process. He focused on the laminar flow regime and on the deposition of SiO.sub.2. A two-dimensional representation of an axisymetric furnace was used to solve the coupled Navier-Stoke equations and multicomponent chemical reactions in both the gas and surface. For a simple reactor this involved a 400-element grid which determined the steady state flow, concentration and deposition rates.
Kalidindi, et al. (April, 1991) J. Electrochem. Soc., 138(4), utilized a Bond Graph approach, which is a lumped parameter and control volume modeling method. They specifically modeled the pyrolysis of tetraethoxysilane for deposition of SiO.sub.2. This modeling approach yields a set of ordinary differential equations which is shown to adequately describe the one-dimensional nature of the reactor diffusion, convection, gas flow and surface reactions. They have only considered the steady-state characteristics of the process.
Extensive work has been done by Jensen and coworkers to study the coupling between physical and chemical phenomena for many realistic reactor conditions. An investigation of flow and heat transfer phenomena was performed by Fotiadis et. al., (1990) using a 50,000 node finite element model (FEM) to solve the momentum, continuity and energy equations. A similar study done by Chehouani and coworkers (Chehouani, et al., (1991) Journal de Physique IV, Colloque C2, suppl. au Journal de Physique II, 1), focused on the pyrolysis of carbon on inductively-heated graphite substrates and used a 2635-node FEM to-establish the influence of operational parameters on the temperature and velocity fields. Ulacia et. al. (1989) Applied Surface Science, 38:370-385, solved the mass-, chemical-species-, momentum-, and energy-continuity equations using a finite volume scheme and several thousand nodes. These authors calculated three-dimensional contours for flow, temperature, pressure, concentrations for various chemical species and deposition rate as a function of process parameters. Arora, et al., (1991) Journal of the Electrochemical Society, 138(5), focused their attention on the surface mechanisms present in the low-pressure deposition of tungsten and included detailed calculations on 65 elementary surface processes in their models.
All these studies have been concerned with steady state aspects of the process. Although Wessbecher, et al., (1992) Thin Solid Films, 207:57-64, focused their attention on non-steady state aspects of simplified reaction kinetics that yield limit cycles, no work that identifies the fundamental dynamics of the entire process has been reported. While the potential of feedback control of CVD has been recognized by Bonnot, et al., (1992) Elsevier Science Publishers B.V., Amsterdam, and by Sheldon, et al., (1991) Mat. Res. Soc. Symp. Proc., Vol. 202, pp. 161-166, these researchers have focused their efforts on the development of in-situ probes and not on the fundamental issues related to developing an appropriate control system.
A number of authors have focused on the development of in-situ sensors. While much of this work is aimed at developing better diagnostic tools, it does provide the basis for developing real-time sensors for feedback control. Important variables that can be fairly easily sensed include temperature (by either thermocouple or optical pyrometry), gas species presence (by mass spectroscopy), and growth rate. More complicated is to resolve the competition between nucleation and growth. Sheldon, et al. (1990) Mat. Res. Soc. Symp. Proc., Vol. 168, have recently utilized light scattering to infer surface roughness, which is expected to be related to the current state of the micro-structure. Their experimental apparatus includes a HeNe 5 mW laser and a linear array of 1024 photo-diodes. The difficulty of this measurement is twofold: experimental difficulties and interpretation of the scattering signal. While some quantitative characterization has been demonstrated for ex-situ measurements, the in-situ measurement is more difficult due to radiation from the substrate. Interpretation of even ex-situ measurements, however, is limited due to the need to assume a model for nucleation/growth and assumptions needed to make the scattering analysis tractable (i.e. that the surface is gaussian). It is known, however, that only in the early stages of growth on a smooth surface are these assumptions valid.
Process development times could be greatly compressed if the CVD process could be controlled with sufficient accuracy to predict deposition thickness profiles on complex shapes, but current theoretical understanding does not support such predictions. In addition, it is difficult to maximize growth rates while maintaining structural integrity (i.e. maintaining desired composition and microstructure). Solutions to these problems are still being sought.